1. Field of the Invention
The present invention relates to a method of making a closed-die forging, particularly of the kind having a flat surface larger than a corresponding flat surface of a blank.
2. Description of the Prior Art
An example of a closed-die forging of the above described kind is shown in FIG. 4. The forging is a constituent part 1 of a knuckle joint for an automotive steering system and in the form of having a spindle 2 and a main body 3 supporting the spindle 2. The main body 3 has an annular flat surface 3a around the spindle 2 and a corner portion 3b at the outer periphery of the flat surface 3a. Such a knuckle joint constituent part 1 is usually produced by a forging process that causes an excess metal or a parting-line flash around a forging. That is, in the forging process, a hot blank is forged in a die in such a way that an excess metal or flash is formed around the main body 3, the arms 4-6, boss 7, etc. in order to assure good die filling. The flash is removed by trimming.
The large flash that is removed by trimming reduces a yield rate of material (i.e., a percentage of forging to blank by weight) to 60% or so. To solve this problem, it is considered to make the knuckle joint constituent part 1 by a closed-die forging process for thereby increasing the yield rate up to 100%.
In the closed-die forging process, such a forging die shown in FIG. 10 is used. The forging die consists of a die 8, an outer punch 9 to be matched with the die 8 and an inner punch 10 slidably installed in the outer punch 9. With the die 8 and the outer punch 9 being held closed, a hot blank 11 heated up to 1250.degree. C. is pushed up by the inner punch 10 with a pressure of 80 Kg/mm.sup.2 and thereby formed into the knuckle joint constituent part 1.
In the above described closed-die forging process, the blank 11 of a diameter smaller than the outer diameter of the flat surface 3a of the knuckle joint constituent part 1 is used in order that the blank 11 is deformed so as to efficiently fill a cavity of the inner punch 10 for forming the spindle 2. Further, in order to prevent defects in the forged product due to buckling of the blank 11, it is necessary that the diameter of the inner punch 10 is smaller than the outer diameter of the flat surface 3a and a little larger than the diameter of the blank 11.
However, the material flow of the blank 11 in the closed-die forging process is restricted considerably as compared with the forging process that causes the flash around the forged product. Due to this, the workpiece is formed with a corner portion 11a at a certain stage before completion of the forging process, i.e., when the inner punch 10 is still moving toward its upper most position as shown in FIG. 11A. As the inner punch 10 further moves toward the upper most position where the blank is completely forged, the corner portion 11a is bent inwards of the workpiece, which can cause a flaw or flaws 12 in the flat surface 3a of the main body 3 of the knuckle joint constituent part 1 as shown in FIG. 11B.